1. Field of the Invention
The present invention relates to a direct methanol fuel cell capable of direct power generation by being supplied with methanol and water as a fuel and with air as oxidation gas. Further, this invention relates to operating conditions, quality control and the like to prevent characteristic degradation of the direct methanol fuel cell, so that the direct methanol fuel cell can generate electricity stably for a long time. This invention particularly relates to a technology of preventing a fuel electrode material of the direct methanol fuel cell to be eluted into the fuel.
2. Related Art
In recent years, measures for environmental problems and resources problem have become important, and as one of the measures, the direct methanol fuel cell is being actively developed. Among the measures, especially the direct methanol fuel cell capable of using methanol to directly generate electricity without reforming and gasifying methanol is simple in structure and easy to achieve miniaturization and a weight saving.
In the direct methanol fuel cell, when a methanol aqueous solution is supplied to the fuel electrode, carbon dioxide gas will be generated by a cell reaction, and used fuel and carbon dioxide gas will be exhausted on the waste fuel side. On the other hand, in the air electrode, when air is applied as an oxidant, the cell reaction will generate water, which will be exhausted from an air exit.
The direct methanol fuel cell operating in this way uses a proton conductive solid polymer electrolyte that is typified by Nafion (registered trademark of E. I. Du Pont de Nemours and Company (Du Pont)) and is composed almost exclusively of perfluorosulfonic acid for the electrolyte membrane. However, it is known that since this electrolyte has a property of allowing methanol that is the fuel to pass through, methanol having passed through the electrolyte increases polarization of the air electrode. Therefore, it is known that methanol in the fuel has its optimum concentration at which characteristics exhibits a maximum, and it has been thought that a concentration of 1 M (mol/dm3) is the best as the concentration of a methanol aqueous solution in the fuel.
For the fuel electrode, a fuel electrode catalyst obtained by making carbon powder having a high specific surface area, such as acetylene black, support platinum and ruthenium of the order of nm is used. This fuel electrode is mixed with PTFE (polytetrafluoroethylene) in order to give hydrophobicity, and further mixed with a proton conductive solid polymer electrolyte of the perfluorosulfonic acid system in order to give proton conductivity to the fuel electrode and to make the electrolyte act as a binder of the fuel electrode catalyst. The air electrode has basically the same structure as the fuel electrode. In the air electrode catalyst, platinum is supported on the carbon powder having a high specific surface area because the air electrode is resistant to suffer CO poisoning.
Outside the fuel and air electrodes, carbon paper or carbon cloth to which PTFE provides hydrophobicity is arranged as a gas diffusion layer also working a charge collector. A membrane electrode assembly (MEA) in which the fuel electrode and the air electrode are formed on a gas diffusion layer, and joined with a proton conductive solid polymer electrolyte membrane such as Nafion is called a five-layered MEA and a membrane electrode assembly (MEA) in which only fuel electrode and air electrode are formed on both sides of the proton conductive solid polymer electrolyte membrane and a gas diffusion layer is separately provided is called a three-layered MEA.
It is normally practiced that a solution of a proton conductive solid polymer electrolyte as, for example, an isopropanol solution is added to the fuel electrode and the air electrode and dried at about 70° C. after membrane formation of the fuel electrode and the air electrode. Moreover, joining of the fuel electrode and the air electrode to the proton conductive solid polymer electrolyte membrane is processed by hot press or by heated roll. The processing is done at a temperature of about 130-140° C. with pressure of about 20-100 kg/cm2.
However, the above-mentioned methanol concentration is derived from a discussion based on a characteristic loss by permeation of methanol through the electrolyte membrane. It has been considered that, only if allowing that characteristic is reduced slightly, a fuel of a high concentration may be used particularly (see Proceedings of The Society of Automotive Engineers of Japan, No. 46-00, 20005062).
Moreover, also in terms of cell operating temperature, since an output characteristic of the direct methanol fuel cell is originally low compared to the solid polymer electrolyte fuel cell, it is customarily operated at a temperature as high as about 90° C. in order to attain a larger-output characteristic. Furthermore, as described in the Journal of the Electrochemical Society (J. Electrochem. Soc.) Vol. 143, No. 1, (1996), L12, recently an operation at even high temperature of about 130° C. is being examined in consideration of applications as a fuel for automobiles.
In addition, investigation to develop a proton conductive solid polymer electrolyte membrane having small methanol permeation other than perfluorosulfonic acid is being carried out, indicating that sulfonated aromatic polymers and the like are promising.